The Supreme Court of Canada says police don’t have the power to arrest anyone acting lawfully simply to prevent a possible outbreak of violence or disorder.

The unanimous decision says such an arrest is not justifiable in cases where someone has not and is not about to commit an offence, and there are less intrusive means available to police to keep the peace.

Today’s ruling stems from a case when Ontario police arrested Randy Fleming at a counter-protest over an Indigenous land dispute in 2009, even though he had not committed any offence.

Fleming successfully argued at trial that police unlawfully detained him, used excessive force and violated his charter right to free expression.

The Ontario Court of Appeal subsequently ruled that police had the power to arrest Fleming, but ordered a new trial on the question of whether excessive force had been used.

In the ruling today, the Supreme Court upholds the original trial judge’s decision, concluding Fleming’s arrest was not authorized and there was no justification for ordering a new trial.